1. Field of the Invention
This invention relates to a magnet pump, and more particularly to a durable magnet pump which comprises means for removing heat typically generated during a non-load operation of the pump and thereby preventing damages possibly caused by such heat from occurring in elements of the pump made of plastic, rubber or the like.
2. Description of the Prior Art
For delivering liquid such as chemical liquid, a relatively low cost pump is utilized which comprises elements made of synthetic resin resistant to such chemical liquid. Since chemical liquid is treated, it is required that a shaft and a casing of the pump be completely sealed. This is because on many occasions such chemical liquid may be expensive and also hazardous to human body. Therefore, as a pump which meets the above requirement, there is known a magnet pump which does not have a shaft sealing member for sealing between the shaft and casing so as to avoid leakage of chemical liquid.
FIG. 1 shows a conventional magnet pump 1a as mentioned above. The magnet pump 1a comprises a casing 3 in which a fixed shaft 6 is accommodated. An impeller 8 is rotatably fitted on the shaft 6. A magnet can 10 is attached to the impeller 8 for accommodating a follower magnet 9a which is adapted to rotate the impeller 8 by transmitting rotations of a motor 28 (not shown in FIG. 1). Also, for rotating this magnet can 10, a driving magnet 9b is arranged in a rotating body 11 fitted on a rotating shaft 29 of the motor 28 at a position proximal to the casing 3. Thus, since the magnet pump 1a as constructed above does not have a shaft sealing member, chemical liquid introduced from an inlet port 20 is completely delivered to an outlet port 21 without any leakage from any part of the pump during a normal operation.
The shaft 6, serving as the rotating central axis for maintaining the rotation of the impeller 8, may be rubbed with the impeller 8 to generate frictional heat. Such frictional heat is cooled down by chemical liquid flow during a normal operation. However, if chemical liquid is not supplied from the inlet port 20 and the impeller 8 rotates without fluid flow, that is, during a non-load operation, the frictional heat is not cooled down and may cause problems, for example, deformation of synthetic resin members. Conventionally, prevention of the frictional heat, that is, the non-load operation of the magnet pump 1a, has been achieved by detecting a load current and stopping the magnet pump 1a by an electrical or pressure control method.
The conventional magnet pump 1a normally employs the impeller 8 and the magnet can 10 made of non-heat resistant material such as synthetic resin. These elements are therefore inherently susceptible to deformation by receiving heat. Also, the wall of the casing 3 is very thin and spacing between the casing and the magnet can 10 is quite narrow so as to obtain a large rotating force of the magnet can 10. Consequently, deformation of these elements causes a crash of the magnet can 10 and/or the impeller 8 with the casing 3, a crack in the casing which prevents the impeller from rotating, and so on, whereby the function of the pump may be lost ultimately.
The applicant has already provided a magnet pump which can eliminate the above-mentioned inconveniences (see Published Japanese Patent Application (Kokai) No. 63-264812), as illustrated in FIG. 2. In a magnet pump 1b shown, frictional heat is generated in portions A and B by the rotation of the impeller 8. To insulate such heat, the magnet pump 1b is provided with a rolling bearing 27 having heat insulating grooves 25, 27, . . . , a rear fixing bearing 4 having a heat insulating groove 4a, a front fixing bearing 5 having heat insulating grooves 5a, 5b, and a shaft 6 having heat insulating grooves 23, 24. The heat generated in the portions A, B is therefore diffused by the heat insulating grooves formed on these bearings 4, 5, 6 and 26 and insulated from the casing 3, the impeller 8, the magnet can 10 and so on, making it possible to prevent deformation, crash and crack from occurring in these elements.
However, even with the magnet pump 1b of the applicant, if it is left in unfavorable operating conditions such as non-load operation, cavitation operation, shutout operation, insufficient load operation (insufficient priming), air lock operation, over-feeding, unstable feeding conditions caused by prerotation effects and so on (these operations or conditions are hereinafter represented by "the non-load operation") for a long period and if such conditions are detected too late, the heat generated in the portions A, B is gradually accumulated therein and conducted to the magnet can 10, the impeller 8 and the casing 3. As a result, the temperature is increased to cause deformation of these elements. Further, such deformation leads to a slack for a short time period between the casing 3 and the shaft 6 and between the rolling bearing 7 and the impeller 8 and/or the magnet can 10. Also, the rotation of the impeller 8 and the magnet can 10 may be deflected, and therefore these elements come into contact with the casing 3, whereby the casing 3 is cracked or deformed. In the worst case, it can be thought that the impeller 8 is stopped, chemical liquid leaks through cracks in the casing 3, and the function performed as the magnet pump will be lost.